Fix SubSub compilation for GHC >= 9.0.1

[jezen]

Resolves #1811.

Related:

• https://stackoverflow.com/questions/73719275/evaluation-of-template-haskell-in-yesod?noredirect=1&lq=1

• Move splices before instance declarations yesodweb.com-content#269

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup N/A
• Added @since declarations to the Haddocks for new, public APIs N/A

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[snoyberg]

Thanks!